Seth Mazzaglia (bottom center) is seen during his video arraignment from the Strafford County jail in Dover, N.H. to the district court in Dover on Monday, Oct. 15, 2012. Mazzaglia was charged with killing Elizabeth "Lizzi" Marriott, a 19-year-old University of New Hampshire student.

Britney Atwood | AP

This undated family photo shows Elizabeth "Lizzi" Marriott. The New Hampshire Attorney Generals office said Saturday, Oct. 13, 2012 that Marriott, who disappeared Tuesday, Oct. 9, 2012 is dead, and a man has been charged with second-degree murder.

Jim Cole | AP

The New Hampshire Marine Patrol continues to search the Piscataqua River near a cliff on Pierce Island for the body Elizabeth "Lizzy" Marriott, a missing University of New Hampshire student on Monday, Oct. 15, 2012 in Portsmouth, N.H. Seth Mazzaglia, a 29-year-old martial arts instructor, was held without bail Monday on a charge of strangling or suffocating Marriott, who vanished a week ago.

DOVER, N.H. — An actor and martial arts instructor accused of killing a female University of New Hampshire student last week was upbeat and described his life as “really good” three days after the woman’s death, an acquaintance said Monday.

Seth Mazzaglia of Dover was charged Saturday with second-degree murder in the death of 19-year-old Elizabeth “Lizzi” Marriott, who vanished a week ago and whose body has yet to be found. He is accused of strangling or suffocating her in his apartment Tuesday night; the search for the body has been focused on Peirce Island in nearby Portsmouth.

Mazzaglia, 29, didn’t speak during a brief arraignment via video feed Monday, and his court-appointed attorneys didn’t object to the prosecutor’s request that he be held without bail.

But Craig Faulkner, who works at a theater company where Mazzaglia had auditioned, said he chatted with Mazzaglia for about 20 minutes on Friday while shopping at Best Buy in Newington. Mazzaglia, who was working in the store’s video game section, told him: “Life is good,” said Faulkner, producing artistic director at Seacoast Repertory Theatre in Portsmouth.

Marriott, of Westborough, Mass., was living with an aunt in Chester, N.H., and commuting to the university in Durham, where she was majoring in marine biology. She was last heard from Oct. 9 when she made plans to visit friends in Dover after attending a class, but never showed up. Her cellphone was last used in Dover that night, according to fliers that family members posted, but authorities said her car was found several miles away in a parking lot on campus in Durham.

“They were familiar with each other,” Senior Assistant Attorney General Jane Young said Monday.

Young said “credible information” has prompted authorities to focus search efforts on the water around the 27-acre island that separates the city of Portsmouth from the Piscataqua River. Marine patrol officials have been using sonar and an underwater camera, she said, but the river’s currents and eddies have hampered their efforts.

“The search in that area may last several more days. They have not exhausted that search,” she said. “We have not discussed an end date. We have discussed continuing this until we find her.”

Authorities in Maine and Massachusetts also have been notified in case her body washes up there, Young said.

Mazzaglia graduated from the University of New Hampshire in 2006 with a degree in theater, Faulkner said. He was known as the “go-to guy” for fight choreography in the area.

Faulkner described Mazzaglia as a quiet, respectful guy but also as someone with a nerdy vibe that made him a bit of “an odd duck.”

“He’s just a little unusual. … I don’t really know how to explain it,” he said. “You don’t meet him and go, ‘Wow, that guy’s a murderer.’”

Faulkner said he never ended up casting Mazzaglia, called him more of a character actor than a leading man. He said Mazzaglia has an advanced black belt designation.

Faulkner said he was later playing the video game Mazzaglia sold him when he heard news of the man’s arrest.

“What I thought about is, I shook his hand two times and if he actually did this. It was one of those, Are you … kidding me moments,” Faulkner said.

Friends and family have described Marriott as a fun-loving, trusting young woman with a wide circle of friends who was active in chorus and a prom queen in high school. She loved animals, volunteered at the New England Aquarium and helped put herself through school by working at Target.

Ken Ziniti, a store manager at the Target store in Greenland, said Marriott was one of the nicest young people he has met.

“Put a smile on everybody’s faces,” he said. “She worked all over the sales floor, always out in front of the guests.”

University of New Hampshire professor David Kaye, who once taught Mazzaglia in an acting class, said Monday that he had been a hardworking student who had a special interest in stage combat. He said people at the school feel for the victim’s family and are shocked and saddened by what has happened.